FIG. 1 shows the typical configuration of prior art raster-scan or imaging plotters. A beam 1, usually a laser beam, is directed at a medium 2, such as a sheet of specially treated photosensitive paper or film. The beam 1 continuously sweeps or scans across the medium 2 as the medium moves in a direction perpendicular to the sweeps of the beam to form a series of parallel lines spanning the width of the medium. These lines are called sweep paths or raster lines and a series of these lines produces a desired image. In a typical plotter or printer a rotating multi-sided (polygon) mirror 4 is used to direct or sweep the beam 1, although other means can be used. The beam 1 is focused by means of a lens 5, normally positioned between the mirror 4 and the medium 2.
Typically, the photo-sensitive medium 2 is mounted on a mounting support device 3 such that the medium can be moved in relation to the beam-mirror lens assembly 6 in a direction perpendicular to the raster lines the beam 1 creates on the medium 2. This relative movement can be accomplished by moving either the beam-mirror-lens assembly 6 or the mounting device 3. The mounting device is often either a flatbed support as discussed in U.S. Pat. No. 4,585,938 issued to Newmann et al. or a rotating drum as discussed in U.S. Pat. No. 4,691,212 issued to Solcz et al.
In typical raster-scan plotters, the energy beam 1 is turned on to create a mark on the medium 2, and turned off to leave a blank space on the medium 2. By turning the beam on and off at predetermined points a contrasting or visible pattern can be created on the medium 2. FIG. 2 shows a magnified detail of a simplified scan. Although typically black, the raster marks 23 are shown here as white with black borders for clarity. In FIG. 2, the beam sweeps from left to right, parallel to the X-axis, turning on 21 and turning off 22 at predetermined points to create a black and white pattern or visible image.
In FIG. 2, the turn on points 21 and the turn off points 22 are marked by crosses. Because the mark that the beam forms on the medium when the beam hits the medium perpendicularly is generally a round dot 26, the raster mark 23 formed by the beam has round or semi-circular ends 25. Each scan or raster mark 23 is very thin. A large number of sweeps or raster lines 24 must be made in order to create a visible pattern. Because of this, the jagged edges caused by the rounded ends 25 of the scan marks 23 are nearly invisible to the naked eye.
Typically, a raster line 24 is divided into a large number of pixels or dots, and the turn-on and turn-off commands for the beam are given coordinates for the pixel where the turn-on or turn-off is to be performed. The Y-coordinate for each pixel represents on which raster line 24 the pixel is located. The X-coordinate represents where on the specific raster line 24 the pixel is located. Of course, a different coordinate system could be used to designate the positions of the pixels.
The prior art has several problems, including the following.
Because of the large sweep angle 7 (see FIG. 1) required to sweep the width of the medium, the pixels or dots created by the beam toward the margins 8 of the medium tend to become oval 27. (See FIG. 2.)
The large sweep angle 7 also requires that large and specially designed focusing lens or f-theta lenses 5 be used. Such large special lenses are very expensive.
Because the beam has a longer distance to travel to the edges of the medium 8 than to the middle 9, the distance the beam travels between the lens and the medium necessarily varies from the fixed focal length of the lens, and thus the outer pixels tend to become defocussed.
In addition, because of the long distance the beam needs to travel, especially at the edges, any jitter present in the mirror assembly 4 becomes magnified.
Further, the prior art systems typically must have large processing and memory requirements, because each sweep path 24 is so long each path has many turn-on and turn-off commands which greatly increase the number of commands that must be processed.